1. The Field of the Invention
The present invention relates to the field of semiconductor optical amplifiers. More particularly, the present invention relates to semiconductor optical amplifiers with optical feedback.
2. The Relevant Technology
One of the ways that information or data is transferred from one location to another is through the use of optical fibers in an optical network. A basic optical fiber network includes a transmitter, an optical fiber and a receiver. The transmitter converts an electrical signal into an optical signal using a laser. The optical signal or light generated by the laser is launched into the optical fiber and transmitted through the optical network until it is received by the receiver. The receiver detects the optical signal generated by the laser and converts the detected optical signal into an electrical signal. The data are then extracted from the electrical signal.
As an optical signal travels in an optical fiber, it often attenuates for various reasons. In order to insure that the optical signal reaches its destination (the receiver, for example), it is often necessary to amplify the optical signal at some point during transmission. In fact, the optical signal may need to be amplified more than once. Traditionally, optical amplification is achieved by regenerating the optical signal at various places in the optical network. Regeneration involves converting the optical signal back to an electrical form, amplifying the electrical signal, and then converting the amplified electrical signal back into an optical signal with a laser.
In addition to being used in regeneration, optical amplifiers can also increase the power level of the optical signal generated by a transmitter laser before the optical signal is coupled to optical fiber. The optical amplifier in this context is often referred to as a “transmitter boost” amplifier.
More recently, optical amplifiers have been created that are able to amplify an optical signal without having to convert the optical signal into an electrical form, as required by conventional regeneration processes. One example of such an optical amplifier is a semiconductor optical amplifier (SOA). SOAs often have an active region that amplifies optical signals via stimulated emission as the optical signal propagates through the active region of the SOA.
One of the goals of optical amplifiers is to provide a linear gain for a range of input power. When an optical amplifier is used in a WDM (Wavelength Division Multiplexing) system, it is also useful to ensure that each channel or wavelength in the WDM signal experiences the same gain.
These goals have proved difficult to achieve. For example, SOAs exhibit a linear gain over a relatively small range of optical input power. As the input optical power begins to increase, the gain of the SOA loses its linearity because of a phenomenon referred to as gain compression or gain saturation. Generally stated, gain compression or saturation refers to the fact that the gain of an SOA drops as the optical power of the input optical signal increases. In other words, the optical gain of a saturated SOA is typically less than the optical gain of an unsaturated SOA. As a result, the linear range of an SOA is limited to a particular power range.
Many efforts have been made to reduce the gain saturation of an SOA and thereby extend the linear range of the SOA, but these attempts (increasing the size of the optical mode, inducing high carrier inversion, for example) are typically associated with practical or fundamental limits. High carrier inversion, for example, can be achieved at high operating current, but at the expense of power consumption and reliability risk. High saturation power can also be achieved by reducing the carrier lifetime, but also at the expense of higher operating current. Increasing the size of the optical mode similarly requires higher material gain to achieve the same amplifier gain, but higher operating current is required. A larger optical mode can also lead to higher loss because the optical field overlaps with more doped (and hence lossy) semiconductor.
In addition to limiting the linear range of an SOA, gain compression or saturation of an SOA can also compromise data transmission, for example, by causing intrachannel interference as well as crosstalk. In other words, at high optical output powers, the gain of an SOA ceases to be linear and the SOA is increasingly subject to crosstalk, intrachannel interference, and the like. What is needed are systems and methods for extending the linear range of the gain of an SOA.